1. Field of the Invention
This invention relates to a rotary kiln plant for burning pulverous raw material such as cement raw meal.
2. Description of the Prior Art
In modern rotary kiln plants of the type contemplated herein, raw material is subjected to sintering and possibly to some final calcining in the kiln. The heat generated in the kiln is utilized in a multi-stage preheater and, if convenient, in a calciner. The temperature of the kiln exit gases which pass from the raw material inlet end of the kiln through a riser pipe and into a lowermost stage of the preheater may be so high as to pose a risk of damage to the kiln structure, particularly at its material inlet end and in the riser pipe. The high temperatures also encourage fine dust particles which are entrained in the hot exit gases of the kiln to cake at these localities especially when the burning raw materials contain chlorine and alkali components.
Several attempts have been made to remedy the problem of overheating of the kiln structure. For example, commonly assigned U.S. Pat. No. 2,750,182 to Petersen relates to a rotary kiln having on the inside of its upper end a dam ring and lifters which cause the raw material to pass into the flow of exit gas. The patent also discloses an opening for supplying atmospheric air to the riser pipe between the kiln and the preheater which cools the gases in the riser pipe.
An improvement of the system disclosed in the Petersen 2,750,182 patent is disclosed in British Patent specification No. 1,100,530 to Elkjaer wherein lifters are built into the dam ring to cause the cooling in the riser pipe to take place by supplying raw material from the penultimate stage of the preheater through a pipe and into the kiln inlet.
U.S. Pat. No. 3,839,058 discloses a construction for preventing cakings in and around the kiln inlet by providing a powerful fan in the exhaust gas outlet outside the kiln, thus ensuring an extra large dust circulation in the upper kiln end.
While the systems disclosed in these references provide effective heat treatment of the cement raw material, none disclose a system which fully utilizes the heat from the hot kiln exit gases while maintaining the temperature of the kiln outlet structure to within acceptable levels as would be desirable. I have invented a rotary kiln plant which avoids such disadvantages.